The present invention relates to a reflection-type liquid crystal display device, and in particular to a reflection-type liquid crystal display device which carries out display without a polarizing plate.
Currently, liquid crystal display devices have been in wide use as a color display with thin and light weight features, for example. In particular, a reflection-type liquid crystal display device, since it does not require a backlight, has a feature which allows the power required for a light source and also a space and weight of the backlight to be reduced. Thus, the reflection-type liquid crystal display device is suitable for devices which are desired to be made light and thin.
As for contrast characteristics of the display, a large decrease in contrast ratio is observed in CRT, etc., in outside day light. Also, as with CRT, in the case where the intensity of surrounding light is much greater than the display light, for example, as in the case of under direct sun light, a large reduction in contrast ratio is inevitable even in a transmissive liquid crystal display device which has been subjected to a low reflection treatment. In contrast, the reflection-type liquid crystal display device can have a display light which is proportional to the quantity of the surrounding light, and therefore is suitable particularly as a portable information terminal device, digital camera, and portable video camera, etc., when they are used outside.
As described, the reflection-type liquid crystal display device has very promising applications in many fields, yet, to this date, no reflection-type liquid crystal display device which can withstand practical use has been developed for the reasons of insufficient performance, for example, in contrast ratio, reflectance, full-color display, and a response to high-definition display and motion images.
The following describes the reflection-type liquid crystal display device in more detail.
A conventional liquid crystal display device using a twist nematic (TN mode) has the arrangement in which a pair of polarizing plates are used, and while it excels in characteristics of contrast ratio and viewing angle dependence, the reflectance is intrinsically low. Also, because a liquid crystal modulation layer and light reflecting layer are separated by the thickness of a substrate, etc., there occurs parallax due to a shift in optical paths of the incident illumination light and outgoing reflected light.
A liquid crystal display device employing a birefringence mode which carries out display by controlling the birefringence of the liquid crystal layer by an electric field (xe2x80x9cECB modexe2x80x9d hereinafter), or a mixed mode in combination of the TN mode and ECB mode allows an arrangement including a single polarizing plate, in which reflectance can be improved while maintaining the advantage of a high contrast ratio of the liquid crystal display device employing only the TN mode. However, such a liquid crystal display device has the problem of poor viewing angle characteristic and coloration in a white display which is caused by wavelength dependence. Further, since a polarizing plate is still used, the utilizing efficiency of the light is inevitably reduced in half or less.
In view of these drawbacks, there has been developed a guest-host liquid crystal element in which the liquid crystal is pigmented in the system with no polarizing plate (xe2x80x9cGHxe2x80x9d hereinafter). However, this device has the problems that it is less reliable by the pigment added and that a high contrast ratio cannot be achieved due to a low dichromatic ratio of the pigment. In particular, a lack of contrast greatly reduces the color purity in a color display using a color filter, which requires the use of a color filter having high color purity. However, the use of such a color filter having high color purity reduces brightness by the color filter provided, posing the problem that the advantage of high brightness of this system as realized by the omission of the polarizing plate is lost.
On these background, there has been developed a liquid crystal display device in the system employing a polymer-dispersed type liquid crystal, for which high brightness and high contrast display can be expected.
This system utilizes the switching characteristic of optical transmissive state and scattering state of the polymer dispersed-type liquid crystal, which states are switched by controlling a voltage applied to the polymer dispersed-type liquid crystal. This system does not use the polarizing plate and the utilizing efficiency of light can be increased. Further, in this system, in evaluation from a view of color quality, a desirable white display can be expected because of relatively small wavelength dependence as compared with that of the ECM mode and because it is free from the problem of absorption profile of the polarizing plate itself (i.e., characteristic of the polarizing plate which absorbs blue light and the incident light becomes yellowish).
For example, Japanese Unexamined Patent Publication No. 104250/1995 (Tokukaihei 7-104250) (Published Date:Apr. 21, 1995) discloses a liquid crystal display device employing such a system, in which polymer dispersed-type liquid crystal is arranged on a black substrate. In this liquid crystal display device, the polymer dispersed-type liquid crystal will be in the scattering state when no voltage is applied, by which the liquid crystal becomes xe2x80x9cnebularxe2x80x9d and a white display is realized, and the polymer dispersed-type liquid crystal will be in the transmissive state when a voltage is applied, by which the black substrate provided underneath becomes visible and a black display is realized, thereby realizing a monochromatic display.
Japanese Unexamined Patent Publication No. 10528/1998 (Tokukaihei 10-10528) (Published Date:Jan. 16, 1998) discloses a reflection-type liquid crystal display device in which a polymer dispersed-type liquid crystal is provided on a reflecting plate whose reflectance is non-uniform and periodic.
Japanese Unexamined Patent Publication No. 90352/1997 (Tokukaihei 9-90352) (Published Date:Apr. 4, 1997) discloses a liquid crystal display element having an arrangement in which a reflecting medium with a tilt angle with respect to a substrate surface is periodically provided on a light absorbing medium, on which is further provided a polymer dispersed-type liquid crystal adjacent to the reflecting medium. In this system too, a white display is realized by utilizing the scattering state of the polymer dispersed-type liquid crystal when no voltage is applied, and a. black display is realized by utilizing the transmissive state of the polymer dispersed-type liquid crystal when a voltage is applied and by absorption of light into the light absorbing medium.
The above publication (Tokukaihei 9-90352) uses the reflecting medium with a shape which is not flat but with a tilt angle with respect to the substrate surface. Japanese Unexamined Patent Publication No. 20290/1998 (Tokukaihei 10-20290) (Published Date:Jan. 23, 1998) discloses a liquid crystal display device including a reflecting medium which is characterized by its pyramid shape or cone shape.
Further, Japanese Unexamined Patent Publication No. 134266/1993 (Tokukaihei 5-134266) (Published Date:May 28, 1993) discloses a display element in which a liquid crystal layer and a polymer material layer are deposited, and which utilizes reflection by interference based on a difference in refractive indices of the two layers and the distance of each layer.
Japanese Unexamined Patent Publication No. 281477/1997 (Tokukaihei 9-281477) (Published Date:Oct. 31, 1997) discloses a reflection-type direct viewing color display device using a hologram which splits incident white light with a predetermined angle by dispersion.
Further, Japanese Unexamined Patent Publication No. 320480/1996 (Tokukaihei 8-320480) (Published Date:Dec. 3, 1996) discloses a color display device with the feature of a light absorbing color filter and a light reflecting color filter deposited.
Japanese Unexamined Patent Publication No. 159778/1995 (Tokukaihei 7-159778) (Published Date:Jun. 23, 1995) discloses a reflection-type color display device which is characterized by the provision of a solar cell under a wavelength selective reflecting medium.
However, in the liquid crystal display device disclosed in the above publication (Tokukaihei 7-104250), only the light scattered backwards from the polymer dispersed-type liquid crystal contributes to the white display, and the light scattered forward is all absorbed by the black substrate, and as a result the utilizing efficiency of light is greatly reduced indeed.
In the reflection-type liquid crystal display device disclosed in the above publication (Tokukaihei 10-10528), the transmitted light through the polymer dispersed-type liquid crystal is not completely absorbed in a black display, and the black display obtained is not dark enough and the contrast is reduced.
These problems are solved by the liquid crystal display element disclosed in the above publication (Tokukaihei 9-90352). This arrangement realizes a desirable black display which is dark enough; however, when it comes to a white display, because the tilt angle of the reflecting plate with respect to the horizontal substrate surface is 42xc2x0 or greater, sufficient brightness cannot be expected unless the scattering efficiency of the liquid crystal layer is superior in order for the incident light to be scattered and to return to the viewer.
That is, in scattering efficiency of a practical liquid crystal layer, scattered light is obtained in the vicinity of light rays transmitting through the liquid crystal layer. However, in the above arrangement, all the light components transmitting through the liquid crystal layer, regardless of the direction of the incident light, are absorbed in the liquid crystal display element and do not emerge from the liquid crystal display element. Thus, most of the scattered light is absorbed in the liquid crystal display element, or even when emerges from the liquid crystal display element, the scattered light does not come back to the viewer. Therefore, sufficient brightness cannot be expected in the above arrangement.
In order to solve this drawback, one can take a measure of, for example, increasing the thickness of the liquid crystal layer. However, this poses another drawback that the driving voltage is increased by the thickness of the liquid crystal layer, and therefore is not practical. There also is an arrangement wherein a reflecting medium is spaced with intervals, yet in this arrangement, the polymer dispersed-type liquid crystal is positioned directly above the light absorbing medium in the intervals of the reflecting medium, and for the same reason as in the above publication (Tokukaihei 7-104250), a white display with sufficient brightness cannot be expected.
Further, in the liquid crystal display device disclosed in the above publication (Tokukaihei 10-20290), because no light absorbing layer is provided, a desirable black display cannot be obtained in a mode with no polarizing plate or dichroic pigment. Furthermore, the above publication does not disclose in detail the limitation of the tilt angle of the reflection plane or the shape of the reflecting plane, other than the disclosure that the reflecting medium is in the form of a pyramid or cone.
Further, in the display element disclosed in the above publication (Tokukaihei 5-134266), in a white display, in order to reflect incident light perpendicular to the display element back to the observer viewing the display device, it is required that the product of the difference in refractive index of the liquid crystal layer and the polymer material layer and the distance of the two layers be narrowed down to the range of a visible light wavelength. Although the above publication does not disclose detailed description of a driving voltage, in general, the driving voltage tends to be high when the distance of the liquid crystal layer and the polymer material layer is narrow, and there is also the problem of high driving voltage. Further, there is also a problem that the reflection wavelength of incident light diagonal to the display element becomes short.
Further, in the reflection-type direct viewing color display device as disclosed in the above publication (Tokukaihei 9-281477), there are required two kinds of holograms for splitting incident light from a certain direction so as to allow the split rays of light to be incident on their respective color filters, which presents the disadvantage in manufacturing accuracy and cost. Also, in using these holograms, the wavelength components which are different from that of the light other than the light incident from a certain direction are reflected, and the color purity of the color filters are reduced.
Further, the color display device as disclosed in the above publication (Tokukaihei 8-320480) does not take into consideration of the incident polar angle of the light reflected by the light reflecting color filter and detailed description thereof is not disclosed. Further, when used as a reflection-type color display device, a color filter will be limited to the combination of, for example, red and cyan which is a complementary color of red, as recited in Examples of the above publication, and therefore this color display device is not suitable for realizing a full-color display.
Meanwhile, in the reflection-type color display device as disclosed in the above publication (Tokukaihei 7-159778), even though it does not include a clear recitation of setting the liquid crystal layer, in order to ensure desirable black display as a color display, as recited in Examples of the above publication, it is required to provide an optical element with good light absorbing characteristic such as a polarizing plate, and the intensity of light incident on the solar cell is reduced indeed.
The present invention was made to solve the foregoing problems and it is an object of the present invention to provide a reflection-type liquid crystal display device having high brightness in a white display, which is capable of clear multi-color display with a high contrast ratio, and which can be manufactured with high productivity.
The inventors of the present application, after extensive research, have come up with an arrangement in which both white display and black display can be realized desirably without a polarizing plate, using a reflection plane inclined with respect to a display plane of the liquid crystal display device. This arrangement is not just limited to the liquid crystal display device employing polymer dispersed-type liquid crystal, but is equally effective for all types of display devices in which switching is made between a transmissive state and a scattering or reflecting state.
In order to achieve the above object, a reflection-type liquid crystal display device of the present invention includes: a pair of substrates; a liquid crystal layer sandwiched between the pair of substrates; one of the pair of substrates or a substrate adjacent to one of the substrates partially having a reflecting property, and a reflection plane being inclined toward the opposite side of an user of the reflection-type liquid crystal display device, and namely the reflection-type liquid crystal display device is arranged such that an inner product of (a) a regular projection vector of a normal vector of the inclined reflection plane onto a display plane and (b) a regular projection vector of a vector in a direction toward the user of the liquid crystal display device is set to be negative, and a tilt angle made by at least a portion of the reflection plane with respect to a horizontal plane of the pair of substrates is not less than xcex8 and less than 2xc3x97xcex8 as defined by an equation
xcex8=xc2xdxc3x97arc sin(n0/n1)xe2x80x83xe2x80x83(1)
where n0 is a refractive index of atmosphere, and n1 is a refractive index of a substance for leveling an inclined plane.
With this arrangement, it is possible to realize a white display with high brightness in a scattering state or reflecting state and with less coloration.
In order to achieve the foregoing object, the reflection-type liquid crystal display device of the present invention includes: a pair of substrates; a liquid crystal layer sandwiched between the pair of substrates; and a reflection plane provided at least partially on one of the pair of substrates or at least partially on a substrate adjacent to one of the pair of substrates, the reflection plane being inclined toward an opposite side of an user, and the reflection-type liquid crystal display device is arranged such that an inner product of (a) a regular projection vector of a normal vector of the reflection plane onto a display plane and (b) a regular projection vector of a vector in a direction from the reflection-type liquid crystal display device toward the user is not more than 0, and a tilt angle made by at least a portion of the reflection plane with respect to a horizontal plane of the pair of substrates is not less than 20xc2x0 and not more than 30xc2x0.
With this arrangement, by setting the tilt angle made by at least a portion of the reflection plane with respect to the substrate horizontal plane within the range of not less than 20xc2x0 and not more than 30xc2x0, it is possible to set an optimum range for a black secure region, i.e., the angle range in which a black display is secured when the liquid crystal layer of the reflection-type liquid crystal display device is in the transmissive state, without using a special material with the refractive index value which is not commonly employed, thus improving both black and white displays.
As a result, by the desirable black and white displays, contrast can be improved, and it is possible to realize a reflection-type liquid crystal display device with high display quality.
In order to achieve the foregoing object, the reflection-type liquid crystal display device of the present invention includes: a substrate including a boundary plane on which external light is incident; a reflection plane for reflecting incident light; and a liquid crystal layer sandwiched between the substrate and the reflection plane, the reflection plane being positioned such that at least a portion of the reflection plane makes an angle of not less than xcex8 and less than 2xc3x97xcex8 with respect to the substrate, where xcex8 is an angle made by the reflection plane with respect to the boundary plane when the reflection plane is positioned such that the incident light perpendicularly incident on the boundary plane of the substrate and reflected off the reflection plane is totally reflected at the boundary plane when the liquid crystal layer is in an transmissive state for the incident light.
With this arrangement, the reflection plane is provided at an angle of not less than xcex8 and less than 2xc3x97xcex8 with respect to the boundary plane, where xcex8 is an angle made by the reflection plane with respect to the boundary plane when the reflection plane is positioned such that the external light perpendicularly incident on the boundary plane, transmitted through the liquid crystal layer, and reflected off the reflection plane is totally reflected at the boundary plane, and that the external light incident from a certain direction emerges from the present reflection-type liquid crystal display device.
With this arrangement, when the liquid crystal layer is in the light transmissive state, incident external light is reflected by the reflection plane so as to limit the direction in which the light emerges from the device.
Namely, by setting the angle made by the boundary plane and the reflection plane in a range of not less than xcex8 and less than 2xc3x97xcex8, the light incident on the boundary plane from an arbitrary direction and reflected off the reflection plane emerges toward an inclined direction of the reflection plane (regular projection direction of a normal direction of the reflection plane with respect to the boundary plane), thus preventing the light from emerging toward the opposite side of the inclined direction of the reflection plane.
This is because, by the definition of xcex8, the light which emerges on the boundary plane from the opposite side of the inclined direction of the reflection plane takes an optical path with a direction of travel opposite to the inclined direction of the reflection plane, from the liquid crystal layer toward the boundary plane to be totally reflected at the boundary plane. However, in the above arrangement, it is less likely that the external light takes such an optical path in the reflection-type liquid crystal display device.
Thus, when the liquid crystal layer is in the transmissive state, when viewing the present reflection-type liquid crystal display device in a perpendicular direction with respect to the boundary plane, no light emerges in this direction, thus obtaining a desirable black display.
Meanwhile, when the liquid crystal layer is in a state of, for example, scattering or reflecting light, the light emerges also in the viewing direction and a white display is obtained. Here, even when the liquid crystal layer is scattering or reflecting light in this manner, it is difficult to make all incident light on the liquid crystal layer to emerge from the boundary plane solely by this scattering or reflecting effect, and some of the light reach the reflection plane.
Thus, there is a need to utilize light which has reached the reflection plane so as to improve brightness in a white display. Here, when the angle made by the reflection plane with respect to the boundary plane is large, the light reflected off the reflection plane concentrates in a direction more towards the parallel direction of the boundary plane on the side of the inclined direction of the reflection plane, and as a result the quantity of the light which directs in the viewing direction in a white display becomes less. Thus, the angle made by the reflection plane with respect to the boundary plane needs to be made smaller. However, in the event where the angle is too small, the light leaks in the viewing direction in a black display and the black display is impaired.
Thus, by setting the angle made by the reflection plane with respect to the boundary plane in accordance with the described arrangement, brightness in a white display can be improved without impairing the black display.
In particular, by making the angle made by the reflection plane with respect to the boundary plane small within the range under the above conditions, i.e., by making the angle to approach xcex8, the brightness in a white display can be maximized within the range which does not impair the black display, and the quantity of the light which emerges in the viewing direction can be increased, thus obtaining a desirable white display as compared with a conventional reflection-type liquid crystal display device. The upper limit of the angle made by the reflection plane with respect to the boundary plane is set to be less than 2xc3x97xcex8. This is because in the scattering efficiency of a practical liquid crystal layer, scattering light is obtained in the vicinity of light rays transmitting through the liquid crystal layer, and in order to obtain a sufficiently bright white display, it is preferable that the light ray incident on the present reflection-type liquid crystal display device and transmitted through the liquid crystal layer emerges from the present reflection-type liquid crystal display device while not impairing the black display. Thus, it is required that the angle made by the reflection plane with respect to the boundary plane is smaller than 2xc3x97xcex8 as defined by the equation (1).
With the described arrangement, when the liquid crystal layer is in the light transmissive state, the light is prevented from emerging in the viewing direction, and when the liquid crystal layer is in the light scattering or reflecting state, more light emerges in the viewing direction, and as a result the brightness in a white display can be maximized without impairing the black display, thereby improving both black and white displays.
As a result, by the desirable black and white displays, contrast can be improved, and it is possible to provide a reflection-type liquid crystal display device with high display quality.
In order to achieve the foregoing object, the reflection-type liquid crystal display device of the present invention includes: a substrate including a boundary plane which is a boundary from an external layer; a liquid crystal layer being parallel to the boundary plane; a transparent film being positioned to face the substrate via the liquid crystal layer and to be parallel to the boundary plane of the substrate; and a reflection plane buried in the transparent film, the reflection plane being positioned such that at least a portion of the reflection plane makes an angle of not less than xcex8 and less than 2xc3x97xcex8 with respect to the boundary plane,
xcex8 being defined to satisfy an equation
xcex8=xc2xdxc3x97arc sin(n0/n1)
where n0 is a refractive index of the external layer and n1 is a refractive index of the transparent film.
With this arrangement, there are provided the liquid crystal layer and transparent film both parallel to the boundary plane, and the reflection plane is buried in the transparent film. At least a portion of the reflection plane is provided to make an angle of not less than xcex8 and less than 2xc3x97xcex8 with respect to the boundary plane (where xcex8 is defined to satisfy the equation (1)).
With this arrangement, when the liquid crystal layer is in the light transmissive state, the light incident from a direction inclined toward the inclined direction of the reflection plane with respect to the normal direction of the boundary plane emerges toward the inclined direction of the reflection plane when emerging from the boundary plane. On the other hand, the light incident from a direction inclined toward the opposite side of the inclined direction of the reflection plane with respect to the normal direction of the boundary plane and reflected by the reflection plane is totally reflected at the boundary plane and does not emerge from the boundary plane.
Thus, when the liquid crystal layer is in a state for accepting a rectilinear propagation of light, the light does not emerge toward the side opposite to the inclined direction of the reflection plane. Thus, as with the reflection-type liquid crystal display device having the described arrangements, when viewing the present reflection-type liquid crystal display device from a direction perpendicular to the boundary plane, no light emerges in the viewing direction, and a desirable black display can be obtained.
On the other hand, when the liquid crystal layer is in a state, for example, for scattering or reflecting light, as with the reflection-type liquid crystal display device having the described arrangements, a white display can be obtained, and by making the angle made by the reflection plane with respect to the boundary plane to approach xcex8, a desirable white display can be obtained. Further, the upper limit of the angle made by the reflection plane with respect to the boundary plane is set to be less than 2xc3x97xcex8. This is because in the scattering efficiency of a practical liquid crystal layer, scattering light is obtained in the vicinity of light rays transmitting through the liquid crystal layer, and in order to obtain a sufficiently bright white display, it is preferable that the light ray incident on the present reflection-type liquid crystal display device and transmitted through the liquid crystal layer emerges from the present reflection-type liquid crystal display device while not impairing the black display. Thus, it is required that the angle made by the reflection plane with respect to the boundary plane is smaller than 2xc3x97xcex8 as defined by the equation (1).
As a result, by the desirable black and white displays, contrast can be improved, and it is possible to provide a reflection-type liquid crystal display device with high display quality.
In order to achieve the foregoing object, the reflection-type liquid crystal display device of the present invention includes: a display plane on which light is incident; a reflection plane for reflecting incident light, the reflection plane being positioned so as to face the display plane; and a liquid crystal layer provided between the display plane and the reflection plane, the reflection plane being inclined with respect to the display plane, and the reflection plane being rotatable within a plane parallel to the display plane.
With this arrangement, the reflection plane for reflecting light incident on the display plane is provided by being inclined with respect to the display plane, and the reflection plane is made rotatable within the plane parallel to the display plane.
Thus, when using the present reflection-type liquid crystal display device, the direction of the reflection plane can be adjusted in accordance with the direction of external light, making it possible to reflect external light to satisfy the condition which is suitable for the reflection plane to carry out display. As a result, in the present reflection-type liquid crystal display device, an optimum display can be obtained in accordance with the conditions in which the device is used.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.